A nonaqueous secondary battery basically comprises a cathode active material, an electrolytic solution and an anode active material comprising a lithium metal or a lithium alloy. In the secondary battery using lithium metal as the anode active material, highly active tree-like lithium metal (dendrite) or mossy lithium metal (moss) is apt to form on the anode during repetition of charging and discharging. When the dendrite or the moss peels off to become in contact with the cathode active material of the battery or when it grows to touch the cathode active material directly, an inner short circuit is produced within the battery.
Recently, as the battery using no lithium metal for an anode active material, proposed are some batteries using carbonaceous materials in which lithium metal or lithium ion can be intercalated and then deintercalated. Such batteries using carbonaceous materials generally have an advantage of increased discharge capacity. Even in the battery, however, lithium metal is deposited and the dendrite is formed on the carbonaceous material when the battery is overcharged or rapidly-charged because the carbonaceous material itself is an electric conductor. Therefore, the amount of the cathode active material is usually lowered so as to prevent the battery from overcharging. In such battery, however, the discharge capacity is not satisfactorily increased due to the restriction of the amount of the active material. Further, since the density of the carbonaceous material is relatively low, its capacity per volume is small. Consequently, the charging-discharging capacity of the battery using such carbonaceous material is restricted for the above two reasons; the restrictions of the amount of the active material and the small capacity per volume.
As known examples of the anode active material other than lithium metal, alloy thereof and carbonaceous material, there can be mentioned TiS.sub.2 in which lithium ion can be intercalated and deintercalated, LiTiS.sub.2 (U.S. Pat. No. 3,983,476), WO.sub.2 having a futile structure (U.S. Pat. No. 4,198,476), iron oxides such as FeO, Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4, and cobalt oxides such as CoO, Co.sub.2 O.sub.3 and Co.sub.3 O.sub.4 (Japanese Patent Provisional Publication No. 3(1991)-291862) and spinel compounds such as Li.sub.x Fe(Fe.sub.2)O.sub.4 (U.S. Pat. No. 4,507,371). Further, a battery in which both anode and cathode active materials are metal calcogenide, e.g., V.sub.2 O.sub.5 or TiS.sub.2 as a cathode active material, and electrochemically synthesized Li.sub.x Fe.sub.2 O.sub.3 as an anode active material, is proposed ((U.S. Pat. No. 4,464,447; Journal of Power Sources vol. 8(1982) pp. 289).
In batteries using the above transition metal oxide or sulfide, the amount of formation of dendrite or moss is less than that of batteries using carbonaceous material. However, each of these known compounds has too high redox potential to Give a nonaqueous secondary battery having high discharge potential (e.g., potential of 3 V or higher (which Generally corresponds to average potential of 2.5 V or higher)), compared with the batteries using carbonaceous material. Further, the above combination also shows a similar characteristics.
As solvents for an electrolytic solution (organic electrolyte) used in such lithium secondary battery, studied is use of nonaqueous solvents of cyclic carbonates such as propylene carbonate (PC), ethylene carbonate (EC) and butylene carbonate (BC); cyclic esters such as .gamma.-butyrolactone (.gamma.-BL) and .gamma.-valerolactone (.gamma.-VL); chain carbonates such as dimethyl carbonate (DME) and diethyl carbonate (DEC); chain esters such as methyl acetate (MA); cyclic ethers such as tetrahydofuran (THF) and dioxolan (DOL) and chain ethers such as dimethoxyethane (DME) and diethyl ether.
As for a lithium salt used as an electrolyte of the electrolytic solution, inorganic salts such as LiClO.sub.4, LiBF.sub.4, LiAsF.sub.6, LiPF.sub.6 and LiCF.sub.3 SO.sub.3 have been studied. An electrolytic solution using LiClO.sub.4 exhibits a high electric conductivity, but it is occasionally detonated under severe conditions such as high temperature. In contrast, the LiCF.sub.3 SO.sub.3 exhibits high stability, and double salts of Lewis acids such as LiBF.sub.4, LiAsF.sub.6 and LiPF.sub.6 show enhanced discharge characteristics. Hence, studies with respect to the LiCF.sub.3 SO.sub.3 and the double salts of Lewis acids have been mainly performed in these days.
As combinations of a solvent and a fluorine-containing lithium salt, the following examples have been proposed.
In a nonaqueous secondary battery using Li metal as an anode active material and fluorine-containing lithium salt as electrolyte, the combination of a mixed solvent of ethylene carbonate and 2-methyl-tetrahydrofuran and LiAsF.sub.6 or LiBF.sub.4 has been proposed (Japanese Patent Provisional Publication No. 59(1984)-96666).
In a nonaqueous secondary battery using cabonaceous material as an anode active material and a fluorine-containing lithium salt as electrolyte, the combination of a mixed solvent of cyclic carbonate and cyclic ester (e.g., PC and .gamma.-BL) and LiXFn wherein X is B, P, As or Sb, and n is 4 or 6 (Japanese Patent Provisional Publication No. 2(1990)-215059) and the combination of a mixed solvent of cyclic esters, esters, cyclic ether and/or ethers (e.g., PC and 2-DME) to which N-methyl-2-pyrrolydone is added and LiPF.sub.6 (Japanese Patent Provisional Publication No. 4(1992)-115471) have been proposed.
However, such batteries using the anode active material (Li metal or carbonaceous material) and the electrolytic solution, are not so improved in formation of the dendrite as to satisfy characteristics such as shelf life, characteristics in charge-discharge cycle, discharging characteristics and safety.
In a nonaqueous secondary battery using transition metal oxide as an anode active material and a fluorine-containing lithium salt as electrolyte, Japanese Patent Provisional Publication No. 3(1991)-291862, which is mentioned previously, describes the combination of propylene carbonate or a mixed solvent of ethylene carbonate and dimethoxyethane (1:1, volume ratio) and LiAsF.sub.6.
Nonaqueous secondary batteries have conventionally employed propylene carbonate as solvent for an electrolytic solution. The above nonaqueous secondary battery uses ethylene carbonate in the amount of 50% in volume other than use of propylene carbonate. The use of ethylene carbonate increase stability of the electrolytic solution. Therefore, the battery scarecely brings about the lowering of the discharge capacity during repeated charging and discharging procedures. However, the battery does not satisfy various characteristics at temperatures lower than room temperature due to its high melting point.